Wednesday, 17 October 2007

Ice belt

I took this half-full tub of pasta sauce out of the freezer to thaw (see Photo). The sauce had been put into the freezer when warm. Why did the ring of ice not stretch from the level of the sauce to the base of the tub?Ewan Drysdale, Head of Chemistry, Kimbolton School, Huntingdon, Cambridgeshire, UK

Looks to me like the bottom section is mostly meat or meatlike material which has a layer of mostly water above it which is taking longer to thaw. Hence the band of frozen water (from atmosphere) showing on the outside of the container indicating a temperature difference inside the container.

The ice is formed by condensation of water vapour on the container. As the stuff in the container freezes more water vapor is condensed out of the air. The heavier stuff on the bottom has hold in the heat longer and takes longer to freeze, which means as the water vapor forms it is re-evaporated from the surface of the container. So you end up with an ice ring that does not extend all the way to the bottom.

did the ice form in the freezer? or when it was sitting out to thaw? if it was in the freezer then i would agree that the meat would stay warmer longer. but if it was sitting out to thaw then i would guess that the counter top would warm the bottom (conduction) faster than the air (convection) could warm the center

A container of small size would likely have a only a small temperature differential between different parts of the surface.

However, I notice that the bottom of the ice ring matches with the bottom of the label on the container, so there may be some effect related to the thermal insulation provided by the label. The area so covered may lose heat more slowly than the bottom.

I suspect also that the label itself may provide a better surface for ice crystals to adhere to and grow on than does the smooth plastic of the container (the small gap may have been filled in from the sides).

When the containe was taken out of the freezer it was a solid lump of gunky ice. It was placed on a relatively warm surface (shelf of a fridge) and proceeded to melt faster at the bottom since solid matter is several orders of magnitude better at conducting heat than air. Hence, the part in conctact with the shelf melts faster and condensation will freeze on the top, colder part.

The container was either put out to thaw or the freezer was left open. The warm, moisture laden, air came into contact with the container, where the water vapor condensed out of the air on contact with the cold container. The paper, being hydrophyllic, trapped the condensed water, which then froze. The plastic, being hydrophobic, did not capture the water and therefore no ice build up on the plastic.

could it be to do with convection currents, because the hotter liquid would have been at the top there would be more heat transfer from the surroudings, radiating more heat away from the top which would leave less enegy nearer the top making it colder, which would make more ice form... thats a mess someone hopefully will prove this wrong within a second, i'm too tired to even think at basic gcse level :(

The icy meal was clearly left in warm moist air. The cold tub cools the air next to it. The air becomes denser and descends causing a reversed convection current. Warm moist air comes from above and condenses on the first cold surface (The top of the frozen contents) by the time the air is at the bottom there is almost no moisture left and the ice is not so thick.

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